In the plants for generating energy of solar origin, the known art is using heliostats—and in particular a field of reflecting mirrors—for concentrating the solar radiation on a determined target, typically a receiver placed at a height using the thermal energy of the solar source that is it houses concentration photovoltaic cells. The heliostats produce the lighting of a big or small area—so-called “impression”—near the target. In an ideal/theoretical system, such area reduces to a point and it corresponds to the focus of the system of heliostats.
The just mentioned known systems with receiver at a height have difficulties and complexity associated indeed to the installation and maintenance at height of a heavy object. Such drawbacks worsen upon increasing the power of the plant for generating energy, as, as it is known, upon increasing such power, even the surface occupied by the field of the heliostats and consequently the height of the related focus and then the positioning height of the receiver increase proportionally.
Such drawbacks are overcome in a configuration alternative to the just mentioned one, already known too and typically called “beam-down”. In the latter configuration a secondary optical system associated to the primary heliostat field is used. The secondary optical system is installed at height and it reflects the radiation concentrated by the primary heliostats towards a receiver positioned at the ground.
In such configuration, the primary focus is the point wherein the rays concentrated by the heliostats in absence of the secondary reflector would converge, whereas the secondary focus is the point wherein the rays converge after reflecting on the secondary reflector and it corresponds, in an ideal/theoretical system, to the site wherein the receiver is placed. In a real system, even the secondary focus corresponds to an area and not to a point, that is it is associated to a “impression”.
Generally, the plants of the “beam-down” type provide as secondary optical system a single reflector in the form of half-ellipsoid or hyperboloid sheet, sometimes implemented by means of a plurality of plane mirrors with small sizes arranged adjacent and so as to be near the wished curvature degree.
As already mentioned above, the light impressions produced by the heliostats or by the secondary reflector are never punctual. This depends both upon the solar divergence, as the rays coming from the sun are not parallel, and upon errors in the curvature of the mirrors implementing the primary and secondary reflectors. In particular, heliostats with big sizes, chosen based upon scale economies, associated to curved secondary reflectors can produce very big impression enlargements on the receiver with respect to the size of the impression obtainable on a receiver placed in the primary focus, that is at height on the top of a dedicated structure.
FIGS. 1 and 2 show schematically an example of “beam down” configuration with secondary reflector with curved reflecting surface, in particular respectively concave and convex surface. In such figures, the secondary reflector is designated with S and M, respectively, an exemplifying primary reflector with O, the primary focus with F1, the point or area of reflection on the secondary reflector with P, the secondary focus with F2 and the receiver with R.
When the secondary reflector is curved, it brings in an optical enlargement of the impression corresponding to the primary focus F1 of the heliostats O, which enlargement is linked to the distance of the reflection point P from the two focuses F1 and F2. In particular, the size of the impression of the rays concentrated by a single primary reflector is enlarged by a quantity proportional to the ratio between the distances P−F2 and P−F1.
FIG. 3 exemplifies the impression corresponding to the secondary focus F2 in case of the convex reflector of FIG. 2.
The enlargement of the concentrated light impression obliges to increase the surface of the receiver R, thus increasing the thermal losses, that is to insert an additional concentrator C near the secondary focus F2. In particular, in the case wherein the receiver is a means for storing thermal energy heated by the solar radiation penetrating the cavity, the increase in the impression of the secondary focus F2 would require an increase in the sizes of the inlet mouth of the cavity itself. On the other side, the interposition of an additional concentrator C between the radiation reflected by the secondary mirror M and the receiver R induces additional optical losses due to the not ideal reflectivity of such additional concentrator.
As said, the curved secondary reflectors are usually approximated with small plane mirrors and this brings in even greater uncertainty about the system precision, potential cause of (additional) enlargement of the impression on the receiver.
A possible solution to the problems deriving from the choice of curved surfaces for the secondary reflector is represented by the use of plane surfaces, as shown in FIGS. 4 to 6. However, even this last choice causes some problems.
By referring to FIG. 4, as known a plane mirror T must be placed at the same distance from the two focuses F1 and F2 in order to avoid an optical enlargement of the concentrated impression. In particular, in order to obtain that all direct rays in F1 converge towards F2 the plane mirror must be positioned at the same distance from F1 and F2 and tilted so as to result orthogonal to the joining line F1-F2. As shown in FIG. 4, the plane mirror T needs a dedicated supporting structure U, even so as to prevent that it shades the receiver at the ground R.
It is also known that the height of the primary focus F1 is proportional to the efficiency in concentrating the radiation and exploiting the soil. However, upon increasing the height of the primary focus F1, the reflection surfaces needed on the plane mirror and thus the loading capability of the related supporting structure increase. Furthermore, a secondary reflector with big sizes would cast a wide shadow on the primary reflectors. In fact, as exemplified in FIG. 5, with a field of heliostats O with circular plan a secondary reflector N parallel to the ground—and by considering vertical the axis F1-F2—would be a very big disc, with ray equal to half of the heliostat field one. As exemplified in FIG. 6, in case of tilted axis F1-F2 the plane reflector, also here designated with T, in any case would have the farest area from the joining line F1-F2 involved by the solar flows considerably higher than the average on the reflector itself, with risks for the mechanical integrity (for example due to differentiated thermal expansions along the extension of the reflecting surface).